IE Circular No. 78-05, Inadvertent Safety Injection During Cooldown

UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
May 23, 1978
IE Circular 78-05
INADVERTENT SAFETY INJECTION DURING COOLDOWN
Salem-1, a four-loop Westinghouse PWR, was being cooled from Mode 3 (hot
standby) to mode 5 (cold shutdown) on January 23, 1978. With one reactor
cooling pump operating, the atmospheric relief valves on all four steam
generators were used to remove heat from the reactor coolant system. When
reactor coolant pressure and average temperature were at 1500 psig and 403
degrees fahrenheit, an inadvertent safety injection occurred due to low
pressure in the steamline from one of the steam generators. Water at 61
degrees fahrenheit was transferred from the refueling water storage tank to
the reactor coolant system resulting in thermal shock to the safety
injection nozzles. Operator action limited the duration of the safety
injection to 2 to 4 1/2 minutes.
The nuclear steam system supplier had informed the licensee that 50 safety
injections using refueling water at 40 degrees fahrenheit would not result
in excessive stress at the safety injection nozzle. Because Salem-1 has had
an unexpectedly large number of inadvertent safety injections, the licensee
is taking actions to reduce the frequency with which they occur.
There are a number of contributing factors which lead to inadvertent safety
injection when the reactor is being cooled from Mode 3 to Mode 5. These
factors include: (1) operation of a single reactor coolant pump instead of
reactor coolant pumps in opposed cooling loops, (2) lack of pressure
recording instruments for the steamlines, and (3) use of atmospheric relief
valves instead of steam dump valves.
The reactor vessel inlet nozzles for the four reactor cooling loops are not
equally spaced on the circumference of the reactor vessel. Two pairs of
inlet nozzles are located on opposite sides of the vessel. The azimuthal
separation between the inlet nozzles of each pair is approximately 45
degrees. With one reactor coolant pump in operation, backflow occurs in the
other three cooling loops and is expected to be greatest in the loop with
the nearest adjacent inlet nozzle. Because of the different flow rates in
the four cooling loops, the task of controlling the cooling rate in each
steam generator is difficult. If the atmospheric relief valves on each
steam generator are positioned alike, the cooling rate will
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May 23, 1978 IE Circular 78-05
be greatest in the steam generator receiving the least reactor coolant flow
and thus the least heat. Therefore, the saturation pressure in that steam
generator falls more rapidly than in the other steam generators. When the
pressure in any steamline is 100 psi less than the pressure in any two of
the other three steamlines, a safety injection occurs.
So that the flow rates will be more nearly equal in the four reactor cooling
loops, the licensee has changed the procedure for cooling from Mode 3 to
Mode 5. The revised procedure requires that two reactor cooling pumps in
opposing reactor coolant loops be in operation when available.
While cooling from Mode 3 to Mode 5, the Salem-1 operator monitors the
pressure in each steamline and adjusts the position of each atmospheric
relief valve as necessary to maintain an equal rate of pressure decrease.
The operator's task is more difficult than necessary because steam generator
pressures are indicated but not recorded on the control panel.
The steam generators can also be cooled using the steam dump valves when the
condenser is available. The steam dump valves are in steamlines connecting
the mixing bottle to the condenser. The mixing bottle is a large diameter
manifold which receives steam from all four steam generators. Use of the
steam dump valves and the mixing bottle, instead of the atmospheric relief
valves, causes heat transfer in each steam generator to be essentially self-
regulating and to tend to maintain the same pressure in each steamline. For
this reason the licensee has changed the procedure for cooling from Mode 3
to Mode 5 to require that the steam dump valves be used when the condenser
is available.
Holders of Operating Licenses for PWRs which have experienced problems with
inadvertent safety injections during cooldown and holders of construction
permits for PWRs should consider actions which would minimize the frequency
of those occurrences.
No written response to this circular is required. If additional information
is needed regarding this matter, please contact the Director of your NRC
Regional Office.
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